The present invention relates to a laser light power control method for use in recording on optical disks, such as a CD-R, CD-RW, DVD-R, DVD-RAM and M0, to control the recording laser light power to follow a predetermined reference power value, and a laser diode driving circuit using such a laser light power control method. More particularly, the present invention relates to an improved technique which can control the recording laser light power from a laser diode with high precision by accurately detecting the laser light power in cases where each recording pulse is provided in a train of divided pulses.
In recording or reproducing data on an optical disk by use of laser light power, it is necessary to control, with high precision, recording or reproducing laser light power that is predetermined depending on the optical disk used. To this end, the so-called ALPC (Automatic Laser Power Control) technique has been used which constantly detects the laser light power during the recording or reproduction operation and performs control to provide the predetermined recording or reproducing laser light power on the basis of the detected power value.
In FIG. 2, there is shown one example of a conventional laser diode driving circuit for optical recording based on such an ALPC technique. Laser diode 10 emits laser light 12 for recording or reproducing data to or from an optical disk. The emitted laser light 12 from the diode 10 is received by a monitor diode 14 provided within an optical pickup, and an output electric current from the monitor diode 14 is converted into a voltage signal via a current-to-voltage converter 16. Peak value detector circuit 18 detects a peak value of the output voltage from the current-to-voltage converter 16; the detected peak value represents laser light power that is actually irradiated onto the optical disk. Offset detector circuit 20 detects a difference or offset between the detected peak value from the peak value detector circuit 18 and a predetermined reference laser light power value and thereby outputs an offset voltage value representative of the offset. Value of an electric current to drive the laser diode 10 is then controlled in accordance with the offset voltage from the offset detector circuit 20 so that the laser light 12 is constantly controlled to provide predetermined recording laser light power.
In FIG. 3, there is shown another example of the conventional laser diode driving circuit, which includes a laser diode 10, monitor diode 14 and current-to-voltage converter 16 similar to those of FIG. 2. In the example of FIG. 3, an output voltage from the current-to-voltage converter 16 is sent to an analog gate circuit 22, where it is sampled in response to a sampling pulse that is generated at predetermined timing corresponding to a recording pulse. The sampled voltage value is held by a hold circuit 24; the thus-held voltage value represents laser light power that is actually irradiated onto the optical disk. Offset detector circuit 20 detects a difference or offset between the voltage value held in the hold circuit 24 and a target laser light power value and thereby outputs an offset voltage value representative of the offset. Value of an electric current to drive the laser diode 10 is then controlled in accordance with the offset voltage from the offset detector circuit 20 so that the laser light 12 is constantly controlled to provide predetermined recording or reproducing power.
Another-type laser diode driving circuit has been known, which is designed to constantly detect a value of a driving current flowing through the laser diode and control the laser-driving current value to follow a predetermined reference value for the recording or reproduction purpose.
Among various known techniques for recording data on an optical disk is the so-called "divided pulse recording", which is characterized by dividing each recording pulse, for forming a single pit on the optical disk, into a train of smaller-width pulses (hereinafter called "divided pulses"). This divided pulse recording technique has the advantage that it can effectively minimize errors in the pit width and length due to excessive heat accumulation.
However, in cases where the laser diode driving circuit of FIG. 2 or 3 is employed in the divided pulse recording, each of the divided pulses tends to have too small a width with the result that the current-to-voltage converter 16 is unable to appropriately follow the pulse frequency, which would result in the output waveform of the converter 16 loosing necessary sharpness, i.e., becoming dull. Such a dull output waveform of the converter 16 would prevent accurate detection of the laser light power, and thus the laser light power could not be controlled with high precision. Further, the laser diode driving circuit of FIG. 3 could not achieve high-speed, high-density recording using the divided pulse recording technique, because of a limited switching speed of the analog gate circuit 22.
Furthermore, with the above-mentioned conventional technique which detects a value of a driving current flowing through the laser diode and controls the driving current value to follow a predetermined reference value for recording or reproduction, it was not possible to control the laser light power with high precision due to the fact that a "driving-current vs. output-laser-lightpower" characteristic of the laser diode would greatly vary due to thermal drift and various other physical changes occurring with the passage of time.